1. Field of the Invention
The present invention relates to a thin film transistor device, a liquid crystal display unit and a manufacturing method for the same, and more particularly, to a thin film transistor device and a liquid crystal display unit which reduces leakage current of a thin film transistor device.
2. Description of the Related Art
In recent years, as a display unit for OA (Office Automation) equipment, various display units using a liquid crystal display unit have been developed. Among various liquid crystal display units, an active matrix type liquid crystal display unit having a thin film transistor (TFT) used as a switching element, has an advantage over conventional displays in that contrast or response speed is not lower much even when the number of scanning lines is increased. For this reason, for high-grade display units for OA equipment and display units for high definition TVs, active matrix type liquid crystal display unit are often used. Also, when an active matrix type liquid crystal display unit has been used as a light valve for a projection-type display unit such as a projector, there is an advantage that large-screen displays can be easily obtained.
When the liquid crystal display unit is used as a light valve of a projection type display unit, the liquid crystal display unit is arranged between a light source and an optical system for projecting light from the light source. At this time, the liquid crystal display unit is arranged in such a manner that the light source is located on the side of an opposite substrate of the liquid crystal display unit and the optical system is located on the side of the thin film semiconductor device array substrate (TFT substrate) of the liquid crystal display unit. Among light with comparatively high luminance that becomes incident from the light source, the liquid crystal display unit controls intensity of the light that transmits through the optical system side on the basis of screen information. More specifically, the liquid crystal display unit switching-drives the thin film transistor and controls an electric field to be applied to the liquid crystal layer for each pixel to change the transmission factor of each pixel and thereby adjusting intensity of the transmitted light. The light that is transmitted by the liquid crystal display unit is enlarged and projected through the optical system having a lens and the like.
Normally, in the active matrix type liquid crystal display unit, a semiconductor layer such as amorphous silicon and polysilicon is used as an active layer of the thin film transistor. When light becomes incident on this active layer, leakage current (light-induced leakage current) due to optical excitation occurs so that the display performance of the liquid crystal display unit is deteriorated because the contrast becomes lower. Particularly when the active matrix type liquid crystal display unit is used as a light valve of the projection-type display unit, since light with high luminance becomes incident on the liquid crystal display unit, an influence due to light-induced leakage current that occurs becomes great. Also, in this case, since in the liquid crystal display unit, not only the light from the light source, but also light reflected by an optical system for projection becomes incident on the active layer of the thin film transistor, the influence due to the light-induced leakage current becomes even greater. In recent years, miniaturization and higher luminance of the projection type display unit has advanced, and the luminance of light that becomes incident on the liquid crystal display unit to be used as the light valve has increased as well. For this reason, the problem of light leakage current has become more serious.
As a technique for reducing the influence due to the light-induced leakage current, for example, Japanese Patent published application 11-204587A and 11-084422A describe a technique for reducing the quantity of light by intercepting return light that becomes incident on the thin film transistor. FIG. 1 shows a cross-section of such a conventional liquid crystal display unit. A pixel portion of this liquid crystal display unit has a first polysilicon film 452a, a siliside film 452b, a second polysilicon film 453, a dielectric film 454, which have been stacked in order on a substrate 451, and a thin film transistor 456 formed thereon.
The substrate 451 is made of, for example, a quartz substrate or a high strain point glass substrate. The first polysilicon film 452a and the siliside film 452b constitute the first light-shielding film 452. The first polysilicon film 452a is formed at a film thickness of about 50 nm on the substrate 451. The siliside film 452b is made of tungsten (W) that is a high-melting metallic material, and is formed at a film thickness of, for example, about 100 nm on the first polysilicon film 452a. The second polysilicon film 453 is constituted as the second light-shielding film, and is formed at film thickness of about 50 nm on the first light-shielding film 452. The dielectric film 454 is formed at a film thickness of about 380 nm on the entire surface of the substrate 451 on the second polysilicon film 453. The thin film transistor 456 includes the polysilicon layer 455 formed on the dielectric film 454 at film thickness of about 65 nm as the active layer, and is fabricated by a similar method to an ordinary fabrication method for thin film transistor for liquid crystal display units.
In the conventional liquid crystal display unit shown in FIG. 1, due to a stacked film composed of the first light-shielding film 452 and the second light-shielding film 453, light that becomes incident on the polysilicon layer (active layer of thin film transistor) 455 is intercepted. In the first light-shielding film 452, light that becomes incident on the active layer 455 of the thin film transistor 456 from the substrate back surface side is intercepted by the siliside film 452b that has a strong optical reflection property. Also, the second light-shielding film 453 consisting of polysilicon film which has a photo-absorption property absorbs the light that has entered between the first light-shielding film 452 and the active layer 455, that could not be intercepted by the siliside film 452b, and reduces the light that becomes incident on the active layer 455.
In the conventional liquid crystal display unit shown in FIG. 1, the dielectric film 454 is formed at film thickness of about 380 nm. In this case, among light that is reflected by the substrate 451 to return, light to be included in an angular difference β between a critical ray of light L11 and a critical ray of light L12 that are shown in FIG. 1 becomes incident on a channel region directly below the gate electrode of the active layer 455. Particularly when a strength of light from the light source increases, the light-shielding effect for the light that is reflected by the substrate 451 to return becomes insufficient even by the second polysilicon film 453 having photo-absorption property. When light becomes incident on a channel region of the active layer 455, light-induced leakage current occurs on the thin film transistor 456 and deteriorates the performance of the liquid crystal display unit. For this reason, particularly when the liquid crystal display unit is used as a light valve of a projection type display unit, a technique for more effectively shielding the light that becomes incident on the active layer 455 of a thin film transistor is needed.